Epilepsy is a common disorder which affects more than 2.5 million people in the United States. New anticonvulsants are needed to treat the more than 50% of patients who continue to have partial seizures on currently available anticonvulsants. This is a prospective, open-label phase II study of a new compound called 534U87. Seizures will be counted during a two month baseline phase on the patient's prior anticonvulsant before 534U87 is gradually added in a three month titration phase. Seizures will then be counted during a three month treatment phase. The first five patients enrolled in the study will be admitted for 24-hours at two week intervals after each of the four or five dosage increases during the titration phase. Serum samples for pharmacokinetic studies will be obtained every 1-2 hours during the 24-hour GCRC admission.